This invention is related to methods for decoding auxiliary information in digital images and, more particularly, to methods for steganographic decoding.
In today""s world, increasing numbers of documents are being scanned in large quantities or are being created electronically and forwarded to another party through electronic means. Often it is of utmost importance that the contents of the document not be seen by third parties who may have access to the message become apparent to the intended recipient.
If a key is needed to decode the information then the intended recipient must have access to the key to see the image or read the text encoded therein. Since the key is independent of the document itself, third parties who may have gained access to the decoding key will also be able to read the document as well. The idea of having a separate key to encode and decode documents is well known in the arts.
This is known as steganography wherein one invisibly embeds a message into an image or signal and provides a key (often referred to as a stego-key) to the intended recipient through a secure channel. Upon receiving the image, the recipient applies the key to the image though a known algorithm to extract the message. A relatively thorough survey of this field of art can be found in xe2x80x9cInformation Hiding: Techniques for Steganography and Digital Watermarkingxe2x80x9d, edited by S. Katzenbeisser and F. A. P. Petitcolas, Artech House Publishers, 2000 wherein, information can be hidden in an image by manipulating the image""s content so that no alteration thereof can be detected by the human visual system but can be detected algorithmically by computer. In other words, the image upon decoding will look unaltered but through computer means it can be determined whether or not the document has been altered somehow between the sender and the recipient""s receipt of the document. Several steganographic applications are known including use for copyright protection wherein the owner""s personal mark is invisibly embedded into the transmitted image.
What is needed in the art and to which the present invention is directed toward a method of using a key to associate a message with the document itself without altering the image contained therein.
What is presented is a method which would then enable a digital image to act as a carrier of hidden or secret information. In particular, the present invention presents a method in which a message is hidden using compact auxiliary information derived from a given image and a given message, such that when the information is applied to the given image, the hidden message is revealed in such a way that neither the image itself nor the auxiliary information is sufficient to reveal the message. Through an application of the present invention, only through the combination of the image and the auxiliary information specific to that image can the hidden or secret message be revealed.
Specifically, a method for decoding an auxiliary message in a digital image, having the steps of first receiving a function F through secure means; then, for each pixel (i,j) in digital image (D) obtaining a Nxc3x97N windowed observation; applying function F to said Nxc3x97N windowed observation; marking a corresponding pixel in message (I) with the value obtained from function F; and then writing out the hidden message.